Section II: Defining Ground Source

Transcription

1 March 2010 Mumbai Delhi Chennai Kolkata Section II: Defining Ground Source Introductory Seminar on Ground Source Heat Pump Systems SAP Programme T002 Funding support from Government of Canada gratefully acknowledged

2 2 Topics in this section History, briefly Types of Geothermal Energy System Boundaries Introduction to Operational Principles Soil Characteristics Terminology / Concept Recap

3 3 History B.C.E. - Egypt / China / Persia / Greece / Rome etc Great civilisations utlize deep springs and thermal vents for baths and for heating buildings 1860 Lord Kelvin establishes variations of ground temperature as a function of depth. (Inspired by the tykhana?) 1912 First design for a subterranean heat exchanger Oct 1st, first north American ground source heat pump in operation.

4 4 Types of Geothermal Energy From Latin: Geo (ground) Therm (heat) Three categories of geothermal energy: High temperature: more than 302 F (150 C) Medium temperature: from 94 F to 302 F (between 90 C and 150 C Low temperature: lower than 194 F (90 C) Note: Medium & Low temperature systems source ground temps that can be used directly.

5 5 Types of Geothermal Energy Ground temperature Depth < 15 m (49.21 ft): Ground temp fluctuates as a function of atmosphere / ambient conditions. Depth ~ 15 m (49.21 ft): Ground temp approximates the annual mean air temperature. Depth > 15 m (49.21 ft): Ground temp increases slowly as a function of depth. This rise is called «geothermal gradient» and is about 2 C per 100 m (~1 F per 100 ft).

6 6 Types of Geothermal Energy Therefore, at 50 feet depth, the ground temp is approximately equal to the mean air temperature. Eg: Montréal (Tair moyen = 6.1 C, Tsol 50 = 8 C) air moyen sol Q: what s the mean ground temperature in Delhi? Mumbai? Chennai?

7 7 So What Is a Geothermal Heat Pump? Basic Schema One of Two Modes

8 8 Operational Principles Works with low to medium subterranean temperatures. Effects mechanical work to exchange heat. - 2 nd law of thermodynamics = energy is not created or destroyed - Ground temps, like a reservoir of cool, feeds the heat pump system; heat pump feeds heat in to the reservoir (one process) - Lower operational temperatures lower system losses / raise efficiencies Designed overall so that utilised (fuel) energy is much less than the transferred energy (COP 3 to 5). Requires only one system to both heat and cool.

9 9 Operational Principles (Con t) This exchange principle is not limited to the ground. Geothermal sources / sinks can vary: closed vertical boreholes, horizontal trenches in soil or water, open loops in ground or surface waters, etc. Geothermal systems seem to reduce heating costs by nearly 65% and of cooling by about 30% over best-case conventional equipment. Configuration changes can dramatically increase these savings Improper installation can dramatically lower these savings Geothermal is a natural / perfect fit with radiant heating / cooling Verified data, like most renewable technologies, is sorely lacking; claimed data is abundant.

10 10 So What Is GSHP? In simple terms, a geothermal system resembles a conventional HVAC system, except that the conventional sources and sinks (i.e. furnace, cooling tower or condensor) are replaced by the ground, groundwaters, or surface waters and may be supplemented by conventionals. For engineers, geothermal systems are high performance integrated thermal transfer systems. For the layman / customer, we often call them heat pumps plus. System boundaries include ground sink, appliance, distribution system(s), and building envelope. Agreed? GeoExchange systems are a national association trademark for ground source or geothermal heat pump systems same technology. These systems do not always require an underground loop.

11 11 Operational Principles Heating Mode Basic Schema Heating Mode

12 12 Operational Principles Heating Mode Geothermal source Compressor Useable heat Plentiful energy Low temperature (8 C) 45 C ( Btu from ground) Btu =? Service cost

13 13 Operational Principles Heating Mode Pumping Energy kw 1 Heat for the building 3 Heating capacity (HC) 2 Heat of Extraction (HE) a.k.a. Heat Absorbed (HA) If COP = unit of electricity + 2 units ground heat = 3 units heat to building ground (main heat source)

14 14 Operational Principles Cooling Mode Basic Schema Cooling Mode

15 15 Operational Principles Cooling Mode Geothermal Source Low absorbent capacity Ground at 28 C Conditioned Space Compressor 30 C 15 C 25 C ( Btu TO the ground) Btu = + Service cost (gas or electric) Btu Extracted

16 16 Operational Principles Cooling Mode Heat from the building 2 kw 1 Pumping Energy Cooling capacity (HE) Heat of Rejection a.k.a. Heat Absorbed (HA) 3 If COP = unit of electricity + 2 units ground heat = 3 units heat to building ground (main heat sink)

17 17 Operational Principles (Con t) Geothermal systems are not compatible with every building type. For example, an energy analysis of this glass-walled building would likely show such elevated cooling loads and heat of rejection that a simple geothermal loop system would not reach profitability. How much loop would be required to cool this building at peak demand? How many seasons would it work until loop failure?

18 18 Soil Characteristics Do not underestimate the importance of site geology, as this will inevitably drive project costs significantly. It s important to get information on the nature of soil and/or rock as this informaiton allows us to: determine best drilling techniques; Precisely define physical properties of the ground such as thermal conductivity, thermal diffusivity, temperature), which reduces uncertainty in loop length calculations. For large projects, hiring a hydrogeologist to execute a thermal conductivity test often pays for itself or is required by the AHJ.

19 19 Soil Characteristics Soils may be classified according to grain size usually measured with a hydromerter and sift table. Sieve and Sediment # Description Size of the Opening (mm) Size of the grating 100 Fine sand Fine sand Medium sand Medium sand Coarse sand Coarse sand Very coarse sand Very coarse sand Very fine gravel Very fine gravel

20 20 Soil Characteristics Thermal conductivity Density Thermal Mass k (variation) k (average) ρ Cp (W/m-K) (W/m-K) (kg/m 3 ) (J/kg-K) Thermal diffusivity α (average) (m 2 /day) Clay 1925 kg/m 3 (15 % water) 1,4-1,9 1, , kg/m 3 (5 % water) 1,0-1,4 1, , kg/m 3 (15 % water) 0,7-1,0 0, , kg/m 3 (5 % water) 0,5-0,9 0, ,045 Sand 1925 kg/m 3 (15 % water) 2,8-3,8 3, , kg/m 3 (5 % water) 2,1-2,3 2, , kg/m 3 (15 % water) 1,0-2,1 1, , kg/m 3 (5 % water) 0,9-1,9 1, ,088 Rocks Granite 2,3-3,7 3, ,107 Limestone 2,4-3,8 3, ,107 Sandstone 2,1-3,5 2, ,088 Moist Shale 1,4-2,4 1, ,075 Dry Shale 1,0-2,1 1, ,065 Gneiss 2,2-3,5 2, ,091 Dolomite 2,8-6,2 4, ,158

21 21 Soil Characteristics Thermal conductivity, k g Comes From: W*m / m² * o K [W/m-K (BTU/hr-ft- o F)] & Btu*ft / hr * ft² * o F Represents the ability to conduct heat and is relevant to loops. High values of k g lead to shorter loops. These values are expressed on an HOURLY Basis Note: More conductive ground means same heat moving along loop faster, and therefore less heat exchange. Thermal diffusivity, α [ft 2 /day (m 2 /day)] Is the ratio of thermal conductivity to volumetric heat capacity, where the volumetric heat capacity is given by the product ρcp ρ is the density [lb/ft 3 (k g /m 3 )] Cp is the specific heat [BTU/lb o F (J/k g - o C)] is the conductivity expressed in a DAILY manner k g Diffusivity is most relevant to loops AND soil around loops.

22 22 Soil Characteristics Effect of different thermal diffusivities Example: This graph shows the temperature rebound in ground, after 24 hours for two distinct thermal conductivities (and a constant conductivity). As you can see, a higher coefficient of diffusivity becomes a greater variation in temperature at a given distance. Vertical Borehole 328 ft (100 m ) Injection of 1 kw over 24 hours Normal ground temp = 50 o F (10 o C)

23 23 Soil Characteristics Effect of different thermal conductivities Example (continued): Best is high conductivity and low diffusivity ratio i.e. most energy comes to pipe from conductivity and leaves the pipe area fastest in heat dumping (i.e. cooling) mode. This also means that conductivity is well matched to ground type. Vertical Borehole 328 ft (100 m ) Injection of 1 kw over 24 hours Normal ground temp = 50 o F (10 o C) Boreholes should be sited at 2x this distance to minimize thermal interference. Q: how do we know what this distance should be for your building?

24 24 Soil Characteristics Precise and complete site evaluation serves to group information and to: Determine loop siting / spacing; Make an informed choice of ground loop system type; Design a system which will take better advantage of the site within its given constraints; Help manage risks; Help control costs; Give a definitive cast to go/no-go decision.

25 25 Soil Characteristics Site Evaluation Report is a summary of results based on examination of existing information. Elements required include: Flow and temperature report on any groundwater - should be both sustainable and usable. Description of stratigraphy, of water quality and of the presence of pollutants. Discussion on the placement of injection and rejection wells and recommendations on the number of wells necessary to achieve required flow. Details on drilling of wells as well as grouting materials.

26 26 Soil Characteristics Important factors in commercial installation: A building footprint of > 30,000 ft 2 (2,787 m 2 ) requires more than one test well (per CGC code / best practice) to determine conductivities. Hydrogeologist or geotechnical engineering services may be required by AHJ and are recommended. The quantity and analysis of groundwater may help determine contamination levels or concerns. In the case of large buildings (but not in detached residential), at least one chromatography test or spectrograph test are required to detect volatile organic compounds.

27 27 Soil Characteristics Site Evaluation Test wells may be used for many reasons: Improve building design, based on realistic estimates of ground resource (ground / soil characteristics can vary considerably from one region to another); help obtain more realistic drilling cost estimates; to adapt design or configuration of ground loops to actual hydrogeological conditions; Satisfy local building / groundwater officials; provide a marketing advantage in bid process to contractor; be utilized for thermal conductivity tests.

28 28 Soil Characteristics If local documentation leads to the conclusion that potable water is improbable on site, wells or test holes on site are not required (according to C448). However, if included in specifications, drillers can better set their prices based on the report. Otherwise, a hydrogeological report will be required that incorporates a chemical analysis of the waters, information on all groundwater contamination and a drilling report. This information must be furnished by a registered geologist. If the system surface area is < 30,000 ft2 (2,787 m2), 1 test well is required (per C448). If the system surface area is > 30,000 pi2 (2,787 m2), 2 test wells are required (per C448). Test wells must reach the same depth as the modelled / expected system boreholes.

29 29 Soil Characteristics Greatest challenges of open loop systems: limitations on groundwater use; respect for standards and laws; costs of hydrogeological analysis; water quality; quantity (and availability) of water; reinjection capacity of local geology. BEFORE proceeding to a site analysis, use all available documentation for verification of the site s hydrogeological potential Question: Should open loop be recommended in the Ganga watershed? Others?

30 30 Terminology See Annex A in CGC s Residential Buyer s Guide, or any CGC Course for a full glossary. Major terms: COP coefficient of performance HE Heat of Extraction; Heat of Rejection GeoExchange geothermal system trademark Ground Source Heat Pump proper technical name for one component of a GSHP system Heat Source & Heat Sink Test wells Site evaluation and Hydrogeological reports